A microbial fuel cell is like any other battery, except that it relies on bacteria to produce its fuel. Construct the microbial fuel cell out of four basic parts. These are an anode, a cathode, a proton-exchange membrane and an external circuit. The anode creates an anaerobic environment in which the bacteria live. The cathode contains a conductive saltwater solution. Simple plastic containers will suffice for these parts of the fuel cell. Install a salt bridge as well, to allow ions to move freely between the anode and the cathode. The electrode pulls electrons from the solution and conducts them through the external circuit. The salt bridge and the proton-exchange membrane transfer protons from waste produced by the bacteria, where they meet with electrons at the cathode. Connect the external circuit to a multimeter to measure the electricity produced, and show your students the results. Alternately, if enough power is generated, use the circuit to light a small, low-power LED bulb.
Use this project to explain how magnets generate electricity, and also how prevalent magnetotactic bacteria are in the world. Ask students to bring in water samples from nearby lakes and rivers, or take a field trip and collect water to use in the project. Magnetotactic or magnetic bacteria are common throughout the world, in both freshwater and saltwater, so many of your samples should yield these bacteria. Place your water samples in small cubes with metal strips on two sides. The strips act as electrodes and cause the bacteria to spin, which generates an electric current. As you did with the microbial fuel cell, connect these cubes to external circuits and demonstrate the potential to generate electricity.
Construct several microbial fuel cells and expose the bacteria within to different food sources. Encourage your class to observe the different extents to which the bacteria within generate electricity. Use a variety of commonly available foods including sugar, starch and organic waste. Record the results over time with your class to determine which, if any, food source encourages better results from your bacteria.
Create several magnetotactic bacteria generators and fill each of them with water from a different source. Because these bacteria are very widespread, you should see some activity in all of your water sources, but population size and density may vary from source to source. Using the electrical output as an indicator, work with your class to determine which water sources have the densest populations. You can also expand on this experiment to demonstrate the differences in biodiversity from one water source to another, and to show that microorganisms experience different levels of success across regional and local habitats.